Today surface acoustic wave device is in wide use as a device having a filtering function in a high frequency circuit of wireless equipment, as an application example, typified by a cellular phone. In such a high frequency circuit of the wireless equipment, an integrated circuit (IC) element having a balanced or differential input/output has been used in recent years.
In contrast, a conventional filter employing surface acoustic wave device (hereafter referred to as SAW filter, as the case may be) has unbalanced terminals in both the input terminal and the output terminal. Therefore, it has been necessary to use a component for unbalanced-to-balanced conversion, which is referred to as balun, or an unbalanced-to-balanced converter constituted of discrete components.
Further, the SAW filter normally has an input/output impedance of 50 Ω. In contrast, an IC for a mixer having a balanced terminal pair, or the like, has a high impedance ranging from 100 Ω to 200 Ω in many cases. In order to connect such an IC with the SAW filter, an impedance conversion circuit has also been needed.
Under such circumstances, an increased number of circuit components has been brought to wireless equipment. Moreover, in order to achieve further miniaturization of such wireless equipment, a space-saving design is required. For this purpose, there has been studied and developed surface acoustic wave device incorporating both the unbalanced-to-balanced conversion function and the impedance conversion function, enabling miniaturization at the same time.
In the course of such study and development, the inventors of the present invention have proposed surface acoustic wave device having both an unbalanced-to-balanced conversion function and an impedance conversion function, disclosed as international application number PCT JP01/05677.
The basic structure of surface acoustic wave device disclosed in the above-mentioned application (PCT JP01/05677) is as illustrated in FIGS. 1 and 2.
FIG. 1 shows a structure of the electrodes constituting a surface acoustic wave (SAW) filter, and FIG. 2 shows surface acoustic wave device 10 in which this electrode structure is formed on a piezoelectric substrate.
An interdigital transducer (IDT) 100 for input and an interdigital transducer (IDT) 200 for output, both formed of a comb structure, are disposed on a propagation path of a surface acoustic wave formed on a piezoelectric substrate 11. Here, these input IDT 100 and output IDT 200 have a relation of reversibility. Accordingly, it is also possible to set reversely the IDT 100 side as an unbalanced output, and the IDT 200 side as balanced inputs, which will also be applicable in the following description.
Additionally, a piezoelectric substrate 11 having the electrode structure shown in FIG. 1 disposed thereon is obtained from a crystal substrate of either LiTaO3 or LiNbO3 cut out at a predetermined angle.
In FIG. 2, an input terminal IN, a grounding terminal GND and output terminals OUT1, OUT2 of the surface acoustic wave device are provided outside a non-illustrated package. Electrode pads formed on piezoelectric substrate 11 are connected to respective terminals through extension leads.
In FIGS. 1, 2, a first electrode finger 101 of comb shape on one side is connected to the input signal terminal IN, and the opposing second electrode finger 102 of comb shape is connected to the ground potential. A length X overlapping between the first electrode finger 101 and the second electrode finger 102 is referred to as an aperture length of IDT 100 for input.
Meanwhile, IDT 200 for output includes interdigital transducers (IDT) 201, 202, which are split into a first split and a second split. Each split has an aperture length X1, X2, which are approximately half in length of the aperture length X, disposed within the range of the aperture length X of IDT 100 for input.
The SAW filter is structured in such a way that an electrode finger on one side of the first split IDT 201 and an electrode finger on one side of the second split IDT 202 are connected to a balanced output terminal pair OUT1, OUT2, respectively. Further, the other electrode fingers of both the first split IDT 201 and the second split IDT 202 are connected in series by a common electrode 203.
With the structure shown in FIGS. 1, 2, it becomes possible that the terminal on the IDT 100 side for input be configured of unbalanced type, while the other terminal on the IDT 200 side for output be configured of balanced type. Further, because of the series connection between the first split IDT 201 and the second split IDT 202 constituting IDT 200 for output, it becomes possible to obtain the impedance of IDT 200 four times the impedance of IDT 100 for input.
Here, particularly the electrode fingers in the first and second split IDT 201, 202 are disposed so that the positions mutually deviate for one cycle, namely one half of the surface acoustic wavelength λ.
Further, in FIG. 1, the common electrode 203 disposed in the connection portion between IDT 201 and IDT 202 is connected through electrode 213 to the second electrode finger 102 connected to the ground potential GND of one electrode side of IDT 100 for input. This enables the common electrode 203 disposed in the portion connecting IDT 201 with IDT 202 to be connected forcedly to the ground potential GND. With such a structure, it becomes possible to obtain satisfactory phase difference balance of a signal on the balanced output terminal pair OUT1, OUT2.
Moreover, the electrode structure shown in FIG. 3 illustrates an application example of the basic electrode structure shown in FIG. 1 applied to double-mode surface acoustic wave device. Namely, based on IDT 100 for input and IDT 200 for output, reflectors REF1, REF2 are provided on the both sides of the multi-IDT (IDT 1–IDT 3, constituting three IDTs in the example shown in FIG. 3), thus forming a double-mode surface acoustic wave structure. The unbalanced input stage and the balanced output stage are connected in cascade connection.
In the structure shown in FIG. 3, the common electrode 203 connecting the split IDT 201 and IDT 202 in series is connected to the ground electrode in the neighboring IDT 204, 205. This also enables satisfactory phase difference balance in the surface acoustic wave device having a double-mode multi-IDT cascade connection structure shown in FIG. 3.
On the premise of employing the surface acoustic wave device having been disclosed in the aforementioned application of the invention by the inventor of the present invention, it is an object of the present invention to provide surface acoustic wave device having improved phase difference balance in balanced input terminals or balanced output terminals, and further preferably suppressing spurious in the pass band, and preventing an increase of an insertion loss.